Heat of adsorption reservoirs provide the possibility of a nearly lossless storage of heat, particularly in the temperature range of up to 250° C., over long periods of time. In particular in connection with the solar thermal heating of buildings in regions of the earth with high seasonal fluctuations of the solarization, i.e. in all regions far away from the equator, a need for such long-term heat reservoirs exists. Here, during the course of the year the highest amount of solar heat from thermal collectors is provided in summer, whereas however the need for thermal heat predominantly exists in winter. In the sense of the development of a sustainable energy supply which is more focused onto regenerative sources of energy the seasonal storage of heat for the heating of buildings is desirable and is a prerequisite for achieving high solar proportions in the solar thermal heating of buildings.
Also, for many other applications the storage of heat in the temperature range of up to ca. 250° C. is an important subject. So e.g. in the case of the decentralized power generation in plants with power-heat coupling (CHP) typically the problem of different temporal need profiles for power and heat arises. For being capable of operating these plants in a power load optimized manner and for being capable of using the generated heat, this heat has to be stored for a certain time, until it is needed. For that, heat reservoirs with high energy density and high efficiency, i.e. low heat losses, are required.
Till today, despite decades of research efforts, heat of adsorption reservoirs have not become accepted on the market. Up to now, primarily, there has been a lack of adsorption materials which provide a large load and heat turnover in the desired temperature range. The zeolites which have often been investigated and used for heat reservoir applications, e.g. zeolites with the structure types LTA and FAU, in particular the commercially available zeolites A, X and Y, typically require for the desorption a driving temperature difference of at least 100° C. between the adsorber and the condenser, thus in the case of a condenser temperature of 35° C. a desorption temperature of at least 135° C. With typical flat plate collectors this temperature cannot be achieved or can only be achieved with very low collector efficiency. Therefore, more expensive evacuated tube collectors or radiation-concentrating collectors are required. With the mentioned zeolites under typical load and unload conditions of a seasonal solar storage system, such as e.g. described in Mittelbach et al., “Solid sorption thermal energy storage for solar heating Systems” (TERRASTOCK 2000, Stuttgart, Aug. 28-Sep. 1, 2000), load turnovers of not higher than 0.18 gram water per gram zeolite are achieved. Thus, based on the density of a bed of the zeolite, reservoir energy densities of up to about 150 kWh/m3 can be achieved (A. Hauer, thesis, TU Berlin 2002, “Beurteilung fester Adsorbentien in offenen Sorptionssystemen für energetische Anwendungen”).
With silica gels comparable energy densities are achieved, but here the main problem is the low usable temperature difference in the case of unloading the reservoir.
Therefore, for the seasonal solar heat storage adsorbents are sought the water adsorption properties of which are between those of typical zeolites and typical silica gels. In particular materials are sought the adsorption isobars of which in the case of a water vapor pressure of about 56 hPa (corresponding to a water reservoir with a temperature of 35° C.) in the temperature range of about 60-110° C. show a load change of at least 0.2 g/g.
Metal-organic frameworks (MOFs) have been developed with respect to a possible use as high temperature hydrogen reservoirs or generally for the sorptive storage of gas (U. Müller, “Metal-organic frameworks-prospective industrial applications”, J. Mater. Chem. 16 (2006), p. 626-636). Due to the high porosity and surface area they are suitable for diverse further fields of application which are traditionally covered by zeolites, such as for example the heterogeneous catalysis or for gas purification.
MOFs are characterized by a modular design. They consist of inorganic polynuclear complexes (cluster) which serve as connectors in the network. Here, the coordination number and the topology of the connector are determined by the coordinating ligands being directed outwardly. As connecting members (linkers) bi-, tri- and multifunctional ligands are used.
With respect to the technical use, due to the good availability and non-toxicity of the metal, in particularly MOFs on the basis of aluminum as metal clusters hold a lot of promise. However, for a lot of applications the low stability with respect to water and particularly water vapor is a problem.
For example, in the case of the storage of methane in an industrial scale residual moisture cannot be prevented. Also, for the use in heat pumps and refrigerating machines on the basis of the adsorption of refrigerants such as for example water, but also alcohols or natural refrigerants (propane, etc.) a stability with respect to water vapor is a prerequisite.
While in the case of the use of water as a refrigerant the stability with respect to water directly arises as a result, also in the case of other refrigerants the stability with respect to water is important, since for example in some process steps the contact with water vapor (atmospheric moisture during the preparation) cannot be avoided. Here, for example the MOF CAU-10-H seems to be very promising, because it exhibits high stability and at the same time good adsorption characteristics.
For the synthesis of MOFs there are different possibilities; most MOFs can be synthesized by solvothermal syntheses. In this case a metal salt and an organic compound are suspended in a solvent or solvent mixture and the reaction mixture is heated in a pressure reactor. This is also the common synthesis for CAU-10-H which can be found in literature (H. Reinsch, M. A. van der Veen, B. Gil, B. Marszalek, T. Verbiest, D. de Vos and N. Stock, Chemistry of Materials, 2013, 25, 17-26): As a reaction mixture a suspension of isophthalic acid (1,3-H2BDC) and Al2(SO4)3*18H2O in DMF and water (1:4 parts) is used. The synthesis is conducted in an autoclave with Teflon liners for 12 h at 135° C. It is reported that during the synthesis in a glass reactor an unknown, crystalline minor phase was obtained. In this literature the synthesis of CAU-10-H was conducted in a 37 ml autoclave. It is mentioned that a scale-up in larger autoclaves is conceivable, but no evidence for that is provided.
Especially water-stable MOFs which should be used as sorption material for heat transformation applications are prepared with water at excess pressure which results in the known, technical large-scale problems:                1. Due to the typical reaction temperatures (>100° C.) it is necessary to work under excess pressure and in corresponding vessels (autoclaves).        2. This complicates the reaction control (no view into the vessels) and increases the costs enormously, particularly in the case, when solvothermal syntheses should be used.        3. The use of glass flasks is hardly possible or only in a limited extent.        
For MIL-160, a MOF which is isostructural to CAU-10-H, a synthesis without the use of pressure in aqueous solution is known in which furan dicarboxylic acid is reacted with aluminum(III) chloride over a period of time of 24 hours. The purification is achieved by means of centrifugation. On the one hand, the use of aluminum(III) chloride which is corrosive and not water-stable is a disadvantage. The purification by means of centrifugation is time-consuming and effortful with respect to the required equipment.
DE 10 2014 215 568 A1 discloses a method for the preparation of an adsorbent out of metal-organic framework structures. In this case it seems to be possible to prepare the structures at atmospheric pressure. A solvent mixture of DMSO and water is used, wherein a relatively low amount of water and a relatively high amount of DMSO (at least 50% by weight) are used. The target is to achieve with the DMSO a boiling point of higher than 100° C. The water in the reaction mixture only has a minor role, and DMSO is assumed to be decisive for the success of the invention. DMSO has the disadvantage that it forms explosive mixtures with some metal salts which are also used in the synthesis of the MOF. The reaction times are in the order of 24 hours and thus, in comparison to the present invention, are extremely long. The document does not disclose the use of an aqueous solution for the synthesis.
US 2011/0282071 A1 discloses photo-active triazole structures. An example for an aromatic dicarboxylic acid is given. But the synthesis of the aromatic dicarboxylic acid in aqueous solution is not disclosed.
DE 10 2006 043 648 A1 teaches a method for the preparation of MOFs as adsorbent. The synthesis is conducted in an organic solvent having a comparatively high boiling point, e.g. in DMF. The reaction time is in the order of 5 days.
WO 2013/186542 A1 describes a method for the production of MOFs in which benzene dicarboxylates are reacted with metal salts in aqueous solution under ambient pressure. For use in the linkers 2,5-dihydroxyterephthalates (straight linkers) and 1,3,5-benzene tricarboxylic acid salts (branching linkers) are presented as carboxylic acid salts which are reacted with salt mixtures composed of Zn/Na or Ni/Na. References to V-shaped linkers in general or isophthalates and their derivates in particular as well as to aluminum salts cannot be found.
DE 10 2005 039 654 A1 relates to mesoporous MOF compounds. It is described to be decisive that in every case the structural features of at least one nitrogen atom in the heteroaromatic of the linker and of at least three substituents X in the form of carboxyl groups (or their thio derivatives) have to be fulfilled. Otherwise the large specific surface areas and the desired mesoporous structure would not be achieved. Thus, it recommended against the use of aromatic dicarboxylic acids. The synthesis is conducted in organic solvent. The use of water is not recommended. With 4 days the reaction times are very long. Desirable would be a method for the preparation of MOFs which                in comparison to the methods of prior art requires less reaction time,        is harmless with respect to the environment,        does not impose special requirements on the working safety (e.g. danger of explosion),        does not require a considerable amount of equipment (e.g. autoclave) and makes MOFs available in very good quality, particularly with high water stability and large specific surface area.        